The invention pertains to improvements in the field of touch tone telephone systems having voicemail recording capability. Specifically, the invention provides the ability to prevent voices having DTMF mimic qualities from causing false DTMF commands to the system from being issued during the process of recording voicemail messages.
Touch tone telephone systems generate DTMF tones when keys on the phone are pressed for such purposes as dialing telephone numbers, giving commands to voicemail systems etc. DTMF tones, generally, are different combinations of audio frequencies generated simultaneously. For example, when the three key is pressed to delete a voicemail message in a telephone system having voicemail messaging capability, typically two tones of different, constant frequency will be generated. The filtering and recognition circuitry in the voicemail command recognition circuitry will detect the two tones and interpret them as a three or the command to delete a voicemail message either just played or in the process of being played.
A problem arises because of the need in touch tone telephone systems with voicemail messaging capability for voice and DTMF tones to be sent on the same communication channel. Human voices generate complex combinations of tones frequently. Certain human voices have the property that at random times combinations of tones are generated which are the same as DTMF tones. Such instances are called DTMF mimics. This is an unfortunate byproduct of the fact that the original designers of touch tone telephones selected DTMF tones to be combinations of tones in the audible range where they must coexist with the audible tones generated by human voices. Now these original selections for DTMF tones have become an industry standard. Thus, designers of telephone systems for use in businesses and homes must include standard DTMF tone generators such that the DTMF tones they generate will be properly interpreted by switching equipment in the central office of the telephone companyxe2x80x94equipment over which the designer of the telephone system connected to the central office has no control.
Had the original designers of touch tone telephone systems selected DTMF tones that were outside the range of tones generated by human voices, the problem would not exist. However, the selection of DTMF tones that were audible does have the advantage that it provides audible feedback to the user through the sidetone heard by the user indicating to that user that the telephone DTMF tone generator is working and that dialing or other commands have been sent.
The specific problem to which the system of the invention provides a solution is the fact that during voicemail message playback DTMF mimics can sometimes occur. When they do occur, results can be unpredictable. For example, a DTMF mimic that commanded xe2x80x9cRepeat the Voicemail Messagexe2x80x9d could cause a voicemail message to be repeated from the beginning each time the point in the message is reached where the DTMF mimic occurs. This would prevent the user from ever hearing the portion of the message past the DTMF mimic. Likewise, if the DTMF mimic is a xe2x80x9cDelete the Voicemail Messagexe2x80x9d command, the message containing the mimic would be deleted as soon as the point in the message where the DTMF mimic occurred.
The prior art teaches systems for detecting DTMF mimics but does not teach the solution provided by the invention for improving the operation of voicemail system by eliminating the unpredictable effects of DTMI: mimics that occur during message playback.
U.S. Pat. No. 4,124,773 entitled xe2x80x9cAudio Storage and Distribution Systemxe2x80x9d to Elkins teaches the use of DTMF signalling to retrieve stored audio signals such as voicemail. It does not address the problem of DTMF mimics in playback or detecting and eliminating DTMF mimics during message recording.
DTMF mimics have been generally a problem in telephone equipment for many years. Before voicemail, the problem of DTMF mimics was handled in standard telephone equipment by only detecting DTMF tones during certain intervals. For example, prior art systems might listen for DTMF tones when a user is placing a call only until enough digits have been entered to make up a complete telephone numbers.
U.S. Pat. No. 5,528,663 filed Jul. 26, 1995 by Locke, et al., and assigned to Sieman-Robin Communications teaches a voice store and forward system with improved DTMF signal detection. This patent is addressed to the problem of xe2x80x9ctalk-offxe2x80x9d which is the term for DTMF detection circuitry accepting a voice signal as a DTMF command thereby creating an error. Other problems addressed by this patent include distortion of DTMF signals when outgoing voice signals mix with inbound DTMF tones in the transhybrid circuit. Because of finite loss in the transhybrid, the DTMF tones will contain additive voice noise thereby interfering with reception of the DTMF signals. The patent notes that one solution to the talk-off problem proposed in the prior art was to make the minimum time for a DTMF tone longer. While this reduces the problem, it does not eliminate it and causes other problems in that some DTMF tone generators have a maximum tone generation time which is shorter than the longer standard. Another solution to talk-off proposed in the prior art was to require a certain minimum power at harmonic frequencies of the DTMF tones before accepting an alleged DTMF command as legitimate. This too did not eliminate the problem. Another solution to talk-off proposed in the prior art was to narrow the bandwidth of the individual DTMF receiver filters. This too did not completely eliminate the problem and can run afoul of the EIA specification for the minimum bandwidth of DTMF filters of 1.5% of the center frequency to account for individual variations in DTMF tone generators. The solution to talk-off proposed by the ""663 Patent involves recognition of DTMF tones using a minimum of processing both power and time. The receiver taught in this prior art reference functions to both identify the DTMF tones as well as provide measurements of the power of its individual components. The receiver outputs signal power, component power and DTMF value. These outputs are provided to a rating algorithm which evaluates the quality of the input signal to determine if it is a legitimate DTMF signal. The algorithm outputs a rating value from 1 to 4 with 4 being the lowest and considered to be non-DTMF. If the algorithm outputs a rating between 1 and 3, the DTMF identification, rating and power are passed to timing/stability algorithm. This algorithm evaluates the stability of the DTMF signal. The algorithm makes its stability determination by evaluating whether the DTMF value or rating or power over time. If any one of these parameters changes over the interval in question, the signal is rejected as not DTMF.
U.S. Pat. No. 5,519,764 filed Mar. 8, 1993 by Pierce, et al., teaches a voice messaging system which scans messages for phrases which might be interpreted as DTMF tones. A record is kept by the computer as to where, when and for how long the DTMF mimic may be occurring. This allows prerecorded or synthesized Voice Response Announcement System messages to be checked to determine if an error caused by a DTMF mimic is about to occur before the message is played. The ""764 patent is employed in telephone system control office systems with Voice Response Announcement Systems. In these systems, a user sends DTMF commands to the central office using his telephone. Sometimes these commands call for messages from the Voice Response Announcement System to be played such as in a voicemail user interface system. A problem arises with the message played back to the user through the central office contains DTMF mimics. If such mimics are present in the outgoing message from the central office, the DTMF receivers in the central office may react by carrying out an undesired action. The ""764 Patent system screens these Voice Response Announcement System messages from DTMF mimics and catalogues them.
U.S. Pat. No. 5,588,053 filed Jan. 11, 1996 by Zheng-yi Xie, teaches a DTMF detector using static and dynamic thresholding. This patent teaches that DTMF signals contain two different sinusoids one of which is in a low band and is one of four standard frequencies and the other of which is in a high band and is one of four standard frequencies. The CCITT and ATandT have established worldwide standards for DTMF detectors. These standards specify the amount of allowable frequency distortion, twist, signal-to-noise ratio, dynamic range, and quadtime. Prior art DTMF detectors using DSP technology have used static-only thresholding where detected tones are compared only to fixed threshold signal levels to ensure valid DTMF detection. This static thresholding only ensures that the detected tones have sufficient energy to warrant detection. The problem with this approach is that the thresholds have to be set high and this limits the dynamic range of the detector and complicates meeting the signal-to-noise ratio specifications requiring that if a DTMF signal has at least the minimum SNR specified in the standard, it must be detected. Other prior art detectors use only dynamic thresholding but that has its own set of problems. The ""053 Patent teaches a DTMF detector which uses both static and dynamic threshold comparisons in a suitably programmed DSP to achieve greater dynamic range and speech immunity.
U.S. Pat. No. 5,408,529 filed Jun. 2, 1993 by Greaves teaches a DTMF detector with improved speech or noise immunity.
Recently, software has become available from third parties for Texas Instruments Digital Signal Processors to do DTMF digit detection and removal. Specification sheets printed from the Texas Instruments website for the DTMF detection and removal software offered by D2 technologies indicate such software is currently available for the TMS320C5x. The webpage has a copyright notice of 1996. If the D2 software was available in 1996, it is possible it is prior art to the DTMF detection and removal functions described herein. Efforts are ongoing to contact D2 and find out whether this software is prior art or not. Another piece of software offered by GAO Research and Consulting Ltd of Ontario, Canada does the same function of DTMF detection and removal and can generate DTMF tones also. The specification for this software also bears a copyright notice date of 1996, so this software may be prior art. In neither case is it clear that these software algorithms are prior art since the DTMF detection and removal function was first offered for sale on Feb. 10, 1997.
Thus, a need has arisen for a voicemail system which can detect DTMF tones during the recording process and either remove them immediately or go back and remove them from the recording later or alter them so that they will not be recognized as DTMF tones upon playback of the voicemail message.
From the above, it is apparent that the prior art does teach DTMF detection and improved immunity to DTMF mimics, but all the methods taught are very complicated. It is apparent that a simple method of avoiding undesired and unpredictable results from DTMF mimics in the playback of voicemail messages is needed.
The invention defines a genus of species all of which either remove or alter data containing DTMF tones in data to be recorded as a voicemail message or which has already been so recorded, so that no DTMF tones will be recognized when the voicemail message is played back. The invention of scanning for DTMF tones or mimics in data to be recorded as a voicemail message is intended to be a stand alone invention and can be employed in any telephone or voicemail/answering machine system. As such, it does not depend upon the particular hardware disclosed herein or the other processes disclosed herein regarding how that hardware works to perform various of the other functions described. The telephone system described herein is intended to be only one example of the many systems in which the invention can be usefully employed. Any system which performs the functions of scanning for DTMF tones or mimics in the data to be recorded as a voicemail message or which has already been recorded as a voicemail message and then either removes the data containing the DTMF tone or mimic or alters it so that it will not be recognized as a DTMF tone upon playback will fall within the genus of the invention. The removal or alteration can be performed either in real time as the data is being recorded or by marking the portions of the recording that have to be further processed and returning later and either removing the data or altering it. The alteration can be performed in any way such as by detecting one of the tones of the two tone DTMF signal and then passing only the offending data to be recorded or already recorded through a digital filter tuned to have a zero at the particular frequency detected so as to remove that tone and destroy the DTMF command. The particular DTMF detection receiver or qualification process is not critical to the invention. Any method of DTMF detection that is currently known in the prior art or which subsequently becomes available can be used for the DTMF detection. Likewise, any method of immediately preventing the recording of the DTMF tones alone or completely deleting the portion of the recording containing the DTMF tones either immediately or marking the limits within which the DTMF commands are found and returning to that portion of the recording later and erasing it or otherwise deleting the DTMF tones will suffice for purposes of practicing the invention. Any species that has both these functions of detecting and deleting either immediately or going back later and deleting is within the genus of the invention.